Metal substrates such as steel sheets, aluminum and aluminum alloys are usually subjected to various types of surface treatment (undercoating) to improve the corrosion resistance, processability, etc.
In recent years, surface-treated steel sheets are required to have higher corrosion resistance, and therefore zinc-based metal plated steel sheets are frequently used as substrates replacing cold rolled steel sheets.
Conventionally, chromate treatment or phosphate treatment is employed for surface treatment of zinc-based metal plated steel sheets.
Chromate treatment has problems with inherent toxicity of chromium compounds. In particular, hexavalent chromium compounds are extremely harmful substances designated as human carcinogens by IARC (International Agency for Research on Cancer Review) and many other public organizations. Specifically stated, chromate treatment has problems with chromate fumes produced during the treatment process, extremely high cost required for waste water disposal equipment, chromic acid dissolved out from chromate treatment coats, and the like.
Phosphate treatment using zinc phosphate, iron phosphate or like phosphate is usually followed by chromate post-treatment, and thus involves the problems with toxicity of chromium compounds. Moreover, phosphate treatment has problems with disposal of waster water which contains reaction accelerators, metal ions and the like owing to phosphate treatment agents, and sludge disposal necessitated by metal ions dissolved out from treated metals.
Japanese Unexamined Patent Publications No. 1983-224174, No. 1985-50179 and No. 1985-50180 disclose coated steel sheets each comprising a zinc-based metal plated steel sheet as a substrate, a chromate coat formed on the substrate and an organic silicate coat formed on the chromate coat. The disclosed coated steel sheets are excellent in corrosion resistance and processability, but have the problems with toxicity of chromium compounds owing to the chromate coat. Without the chromate coat, the coated steel sheets have insufficient corrosion resistance.
Aluminum or aluminum alloy substrates are also subjected to various types of surface treatment (undercoating) in many cases, to improve the corrosion resistance, hydrophilicity and other properties.
Generally, fins in heat exchangers for air conditioners are made of aluminum or aluminum alloy substrates which are light in weight and excellent in processability and thermal conductivity. In air conditioner heat exchangers, water condenses into droplets and forms water bridges between the fins during cooling operation. The bridges narrow the passageway for air and increase the resistance to air passage, thus causing problems such as power loss, noise, water splashing, etc.
To solve these problems, surfaces of the aluminum or aluminum alloy fins are subjected to boehmite treatment, water glass coating, aqueous polymer coating or like hydrophilizing treatment for preventing bridge formation. However, in a highly corrosive environment, the hydrophilized aluminum or aluminum alloy fins are corroded within a few months or so, partly because of the hydrophilicity of the treatment coat.
To prevent corrosion of the fins, chromate treatment is often employed for undercoating of aluminum or aluminum alloy substrates, since chromate treatment has the advantages of providing good corrosion resistance with low cost. However, chromate treatment is accompanied by the problems with toxicity of chromium compounds as described above.
As chromate-free undercoating materials and undercoating processes, Japanese Unexamined Patent Publication No. 1979-24232 discloses treatment of an aluminum surface with an acid solution comprising a titanium salt, hydrogen peroxide and condensed phosphoric acid; Japanese Unexamined Patent Publication No. 1979-160527 discloses treatment of an aluminum surface with an aqueous alkaline solution containing titanium ions and a complexing agent, followed by water washing and treatment with an aqueous solution of an acid such as phosphoric acid; Japanese Unexamined Patent Publication No. 1997-20984 discloses an aluminum surface treating agent comprising phosphoric acid ions, a titanium compound and a fluoride; and Japanese Unexamined Patent Publication No. 1997-143752 discloses an aluminum-based metal surface treating agent comprising a condensed phosphate, titanium salt, fluoride and phosphite.
However, these undercoating materials and processes utilizing titanium compounds have the problems such as insufficient stability of the undercoating materials, lower corrosion resistance of the coat than a chromate treatment coat, insufficient hydrophilicity and insufficient durability of the coat.
In view of the above state of the art, there are demands for an inorganic film-forming material which is useful as an undercoating material for metal substrates such as steel sheets, aluminum, aluminum alloys or the like, and which is capable of forming a film excellent in corrosion resistance and other properties without causing toxicity problems.